A DMOS (Double diffused MOS) transistor is a type of MOSFET (Metal Oxide Semiconductor Field Effect Transistor) that uses two sequential diffusion steps aligned to the same edge to form the channel region of the transistor. DMOS transistors are often high current devices for low and high voltages, used either as discrete transistors or as components in power integrated circuits. DMOS transistors can provide high current per unit area with a low forward voltage drop.
One particular type of DMOS transistor is a so-called trench DMOS transistor in which the channel is present on the sidewall of a trench, with the gate formed in the trench, which extends from the source towards the drain. The trench gate, which is lined with a thin oxide layer and filled with polysilicon, allows less constricted current flow than a planar gate DMOS transistor structure and thereby provides lower values of specific on-resistance.
However, a conventional method of making such a trench DMOS field effect transistor requires a five to six masks process that is expensive and time consuming.
It is within this context that embodiments of the present invention arise.